1. Field of the Invention
The present invention relates to a step zoom lens camera in which a focusing operation is carried out by controlling the angular displacement of a cam ring having a cam groove.
2. Description of the Related Art
In classical zoom lenses, upon zooming, a plurality of variable power lens groups are moved along predetermined zoom path, so that the focal length can be varied without moving the focal position. In a shutter release operation, a focusing lens group is moved in accordance with the object distance. The focusing lens group is either provided separately from the variable power lens groups or is formed by one of the variable power lens groups. Classical zoom lenses of this kind have been used for a mechanical zoom lens in which a cam ring having cam grooves is manually or electrically rotated in a stepless fashion.
A step zoom lens has been used for a lens in which the angular displacement of the cam ring with cam grooves is pulse-controlled. In such a step zoom lens, the focal length from the telephoto extremity to the wide angle extremity is divided into a plurality of (finite number of) focal length steps. The angular displacement of the cam ring is controlled at each focal length step so as to focus on an object of any distance between the infinite object distance and the minimum (closest) object distance, during the focusing operation while the zooming operation is carried out. The angular distance of the cam ring is pulse-controlled so that the object is in-focus in accordance with the focal length belonging to a specific focal length step determined in accordance with object distance data.
In the step zoom lens, if a focusing table which shows the relationship between the object distance and the focal length is stored, it is possible to calculate the angular displacement of the cam ring with respect to the current angular position thereof when the shutter is released, so as to coincide with the object distance. Therefore, the focusing can be carried out, regardless of the profile of the cam groove. Accordingly, each step portion thereof (corresponding to a focal length step) is formed linearly in a conventional cam groove. However, if each step portion of the cam groove is made linear (or substantially linear), the relationship of the angular displacement of the cam ring and the movement of the image surface is non-linear, and thus, control thereof is difficult. For example, in the step zoom lens in which the pulse-control of the angular displacement of the cam ring can be performed, the zooming adjustment (to make the focal position at each focal length identical) or the backfocus adjustment (to make the focal position coincident with the image pickup surface (film surface)) can be carried out by setting (adjusting) the angular displacement of the cam ring. If the relationship between the angular displacement of the cam ring and the movement of the image surface in each focal length step is non-linear, the calculation of the number of pulses including backfocus adjustment during the focusing operation is made particularly complicated. If the pulse calculation is complicated, the burden of the control system of the CPU or memory is increased.
It is an object of the present invention to provide a reliable step zoom lens camera in which the burden of the control system during the focusing operation or adjustment is reduced and the focusing can be precisely carried out.
To achieve the object, according to the present invention, a step zoom lens camera is provided, having a zoom lens and a cam ring which is rotated to vary the focal length of the zoom lens, the cam ring being provided with a cam groove including a finite number of step portions which correspond to a plurality of focal length steps defined between a telephoto extremity and a wide angle extremity of the zoom lens, so that an object from an infinite distance to a closest distance can be focused on by changing the rotation angle of the cam ring in each the focal length step while varying the focal length of the zoom lens. The step zoom lens camera includes a focusing lens group which serves also as a variable power lens group of the zoom lens, wherein the focusing lens group is moved in the optical axis direction, while keeping a linear relationship with respect to the rotation angle of the cam ring in each focal length step, in accordance with the rotation of the cam ring; and an adjusting lens group, serving also as another variable power lens group of the zoom lens, which is guided by the cam groove and is moved in the optical axis direction in a non-linear relationship with respect to the rotation angle of the cam ring in each focal length step, in accordance with the rotation of the cam ring. The step portions are each provided with a path which enables the adjusting lens group to move so as to provide a linear relationship between the rotation angle of the cam ring (i.e., amount of movement of the focusing lens group) and amount of movement of the focal position of an infinite distance object.
In an embodiment, the cam groove is provided with a path which enables the adjusting lens group to move wherein a rotational angular displacement of the cam ring and the displacement of the focal position are equal in each of the focal length step portions.
Preferably, a position of the focusing lens group is adjustable in the optical axis direction, relative to the cam ring while the cam ring is stationary, so that a zooming adjustment to make the focal position equal in each focal length step is carried out by adjusting the position of the focusing lens group, wherein the focusing lens group and the adjusting lens group are moved along predetermined paths in accordance with the rotation of the cam ring during the focusing operation, in order to perform a backfocus adjustment to thereby make the focal position equal to an image surface of the step zoom lens camera.
In an embodiment, the zoom lens includes two lens groups including the focusing lens group and the adjusting lens group, wherein the focusing lens group includes a front lens group located on the object side, and the adjusting lens group includes a rear lens group located on the image surface side.
In an embodiment, a cam groove for guiding the adjusting lens group and a female helicoid are provided on inner peripheral surface of the cam ring. The focusing lens group is supported by a movable barrel having a male helicoid which is engaged with the female helicoid, the movable barrel being guided to move linearly in the optical axis direction.
Preferably, a mechanical adjusting device is further provided for adjusting the position of the focusing lens group in the optical axis direction with respect to the movable barrel.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-264111 (filed on Aug. 31, 2000) which is expressly incorporated herein by reference in its entirety.